The national database of the use of stallions, supplied by the Department of Animal Registration and Breeding Organization of the Hungarian National Food Safety Authority was analyzed for the purpose of this research. 680 foaling data of 7 horse breeds was processed during the work. The factors effecting gestation length were examined by univariate analysis of variance GLM. Breed, month of mating, method of fertilization, age of the mare, and sex of the foal were considered as fixed effects, and the sire was used as random effect in the study. The population genetic parameters of gestation length and the breeding value of sires were also estimated. The overall mean value of gestation length was 334.1±2.62 days. The longest gestation length 336.2 days was detected in the Kisberi breed while the Shagya had the shortest 333.3 days. Differences between the breeds were not significant. The effect of month of mating and effect of sex of foal on the gestation length were statistically proven P